BCKDHA and maple syrup urine disease: The classic form of MSUD (cMSUD) is mostly caused by mutations in BCKDHA, BCKDHB and DBT. To date, at least 93 mutations in BCKDHB have been described in MSUD patients, according to the Human Gene Mutation Database (www.hgmd.cf.ac.uk), most of which are missense mutations.